How to overcome warp in HDPE molding?

If you haven't already considered pack pressure, pack/hold time and cushion size, you might want to take a look at it. Most non-filled, semi-crystalline materials will have warp problems if they are over-packed. Of course, part geometry, wall thickness, etc. will play a part in warping of the part. But I would start with checking the fundamentals of the setup; the pack/hold dynamics would be a good place to start.

I have summarized the advice I got with a few remarks of my own based on the experience gathered:

Perform a mold flow analysis on the part especially concerning residual stresses, gate size and location and cooling channel location. This process is still on going so I really can't comment on the results.
Heating the mold helped considerably.
Differential heating also helped (20C temperature difference between mold sides).
Low pack and hold pressures.
Use of fillers actually made things worse.
Cooling on fixtures helped.
Use of MDPE helped.
Foaming didn't help much.
Making sure mold temperature is constant; this was never an issue so I can't comment.
Making sure wall thickness is constant; this was never an issue so I can't comment.
Annealing helped.
Increase temperatures in the central nozzles were never tried.
Nucleating agents didn't help much.

HDPE warpage mainly will depends on the tool temperature and how this temperature is uniform or not. Also you must to analyze the gate, because depends on position, you may have a unsolved problem, unless you change the gate position. In the end if you see carefully the barrel parameter, like screw speed temperature back pressure and other things, the HDPE must to have a very uniform temperature before go into the mold cavity. if you did everything, and didn't work, maybe you should put some filled (CJAVE3), around 10% or 15% Good lucky!

- - - -> by: Paulo

The geometry of the part as well as the direction of the cooling lines also are important when working with HDPE.
Ensure that the mold has no cold spots due to cooling Hot spots too are a problem.
Perhaps a mold flow analysis will prompt a direction of designing the inserts with minimum warpage.

- - - -> by: William